Inorganic and organic chlorides present in even small quantities in hydrocarbon streams can upset the process conditions either by poisoning the catalyst or by causing corrosion of the equipment. For example: sever corrosion is observed in crude top column, hydrotreater reactor top and downstream circuit including surge drum, high temperature exchangers and transfer lines especially along the elbow/joint sections. Generally pitting type of corrosion is observed in these equipments, suggesting that chlorides are the main cause of corrosion. Among chlorides, hydrogen chloride and ammonium chloride have the highest potential to cause corrosion. In the hydrotreaters, the organic chlorides also get converted to inorganic chloride i.e. hydrogen chloride, therefore worsening the corrosion problem.
The hydrocarbon stream coming from crude distillation unit has significant amounts of inorganic and organic chlorides. For example: the gas oil produced in atmospheric distillation may contain up to 25 ppm chlorides while the light vacuum gas oil produced in the top section of vacuum distillation unit may contain up to 50 ppm chlorides. These chlorides are mainly produced by hydrolysis of chloride salts of magnesium and calcium metals. These metal salts are present in the crude oil and carried to the distillation unit due to insufficient desalting. Thus, both the inorganic and organic chlorides are detrimental to the hydrocarbon processing units, especially the hydrotreaters, and must be removed from the hydrocarbon stream prior to processing.
Commonly, an adsorbent or a catalyst is used to remove the chlorides from the hydrocarbon streams. U.S. Pat. No. 3,864,243 discloses use of bauxite as a chloride adsorbent, where the adsorbent is dehydrated at 425-650° C. before use. U.S. Pat. No. 3,935,295 discloses the use of calcium and zinc oxide adsorbent for removal of inorganic chlorides. U.S. Pat. No. 4,713,413 discloses the use of alumina adsorbent at 20° C. for removing organic chlorides. U.S. Pat. No. 5,107,061 discloses the use of crystalline molecular sieve Zeolite X in soda form for removal of organic chlorides from a hydrocarbon stream. U.S. Pat. No. 5,595,648 and U.S. Pat. No. 5,645,713 disclose the use of low surface area solid caustic bed for removing chlorides from hydrocarbon streams. U.S. Pat. No. 5,614,644 discloses the use of copper containing scavenger material for removing organic chlorides from hydrocarbon streams and U.S. Pat. No. 6,060,033 discloses, the use of alkali metal oxide loaded on alumina for removing inorganic chlorides from hydrocarbon streams.
Use of the afore-mentioned chloride adsorbents for removing chlorides from heavy hydrocarbon streams such as vacuum gas oil, or coker oil is unfeasible due to their high viscosities and high pour points and presence of small amounts of asphaltenic materials. These properties of the heavy hydrocarbon streams cause following problems when an adsorbent is used: high delta pressure is required across the adsorbent bed, higher temperature conditions are required for adsorption, difficulty in regeneration of the adsorbent, lower chloride loading capacity and lower chloride removal efficiency.
Also, catalysts have been used in the past for converting the organic chlorides to inorganic chlorides. U.S. Pat. No. 3,892,818 uses rhodium catalyst to convert pure organic chlorides to hydrogen chloride, where the catalyst contains 0.1 wt % rhodium and the reaction is carried out at about 250° C. U.S. Pat. No. 4,721,824 discloses the use of magnesium oxide and binder for catalytic removal of organic chlorides from hydrocarbon streams. U.S. Pat. No. 5,371,313 discloses the use of calcium oxide at 130-170° C. for removal of tert butyl chloride. The above-mentioned processes are suitable for light hydrocarbon streams (<C20) and further treatment of the hydrocarbon stream is required to remove the inorganic chlorides produced thereof.
Some other known processes use additives to remove inorganic chlorides or reduce the corrosion impact. U.S. Pat. No. 5,269,908 discloses the addition of an inert gas such as steam, nitrogen, organic gases or natural gas, for reducing ammonium chloride deposition. U.S. Pat. No. 5,387,733 discloses the use of non-filming polyamine additive for inhibition and removal of ammonium chloride deposits in hydrocarbon processing units. U.S. Pat. No. 5,558,768 discloses the use of a non-ionic surfactant (copolymer of ethylene oxide and propylene oxide) for removal of chlorides from crude oil. The above-listed methods involve use of additive/catalyst and require a subsequent treatment for removal or deactivation of the additive/catalyst.
Further, some known processes use distillation or stripping, optionally in the presence of an additive, to remove impurities from hydrocarbon streams. A process for separating lighter components such as hydrogen, hydrogen sulfide, ammonia and hydrocarbons having less than 11 carbon atoms, from a heavier heating oil, by employing two stripping mediums is disclosed in U.S. Pat. No. 5,141,630. The first stripping medium removes/separates the lighter components and the second stripping medium removes or separates residue of the first stripping medium and any light components remaining in the feedstock. The stripping temperature is maintained between 200-750° F. (93-400° C.) and the stripping pressure is between 0-200 psig (0-14 bars). The first stripping medium is typically selected from hydrogen, methane, propane, steam or other inert gas, and the second stripping medium is typically nitrogen gas. According to the process disclosed in U.S. Pat. No. 5,141,630, steam is not a preferred stripping medium at the said process conditions since it saturates the stripped product with water and a subsequent drying step is necessary to remove the moisture. Also, this process is not suitable for removing chlorides from the heavy oil.
Another known process to remove chlorides is disclosed in US2004238405 which discusses converting the chlorides to volatile compounds by treatment of the oil stream with an additive and a stabilizer; volatile compounds can then be removed by stripping. This process is suitable for treating crude oil. Still another process is disclosed in U.S. Pat. No. 4,992,210 which comprises using an organic amine and potassium hydroxide in a water soluble solvent to desalt corrosive contaminants such as magnesium chloride, sodium chloride, calcium chloride and organic acids from crude oil. Yet another method is disclosed in GB1105287 and GB724266 which comprises preventing corrosion by using corrosion inhibitors. The method disclosed in GB1105287 involves preventing corrosion of metallic petroleum refining equipment by admixing a base such as sodium hydroxide or potassium hydroxide, to crude oil, to restrict the formation of corrosive hydrochloride and hydrogen sulfide. GB724266 discloses use of guanidine or a derivative of guanidine to reduce corrosion of the distillation apparatus during steam distillation of hydrocarbon oils.
All the above-listed known processes are used for treating light hydrocarbon fractions or crude oil. Heavy hydrocarbon streams are difficult to treat due to their high viscosity and pour point. The chloride removal from such streams is further difficult when concentration of the impurity is in the range of few ppm.
Still furthermore, all the presently known processes involve distillation which results in the removal of large amount of hydrocarbons while removing the chlorides. There is envisaged in accordance with the present disclosure a process for removal of chlorides that is based on stripping of the hydrocarbon and that involves minimum distillation.